Frame It Up
The simplest frame uses hay bales: Just arrange four bales of hay or straw into a square shape to make the sides of your coldframe. Put your glass or plastic cover on top of the bales. (Use the straw for mulch next spring, after you disassemble the frame.)
If you can’t get bales of hay or straw where you live, you can use other materials for your side walls. Cinder blocks are a good alternative; just be sure to turn them on their sides so the holes point up and down. Otherwise, air will pass through. Cover the top holes to keep the structure warmer.
To make a more permanent and easily vented structure, build the sides from wood and attach your top to it with hinges. Cedar, cypress, and redwood are naturally rot-resistant, but you can use almost any kind of wood—as long as it isn’t pressure treated (CCA, Wolmanized, and so on). Pressure-treated wood contains highly toxic substances, including arsenic. Secure the pieces of wood with elbow braces at each corner, glued and then screwed in with two 1- or 1 1⁄2-inch galvanized screws.
If you garden in an extremely cold area, you’ll need a more permanent and better insulated coldframe. Jan Scheefer, a high-altitude gardener in Gunnison, Colorado, made her coldframe walls out of 6-inch-thick poured concrete, which she painted black to absorb solar heat. She capped the frame with corrugated fiberglass framed with pine 2-by-4s.
Stone and mortar walls are another option. Building stone walls requires more labor and know-how than pouring concrete, but they can be much less expensive if you happen to have stone on your property.
By digging a pit beneath your coldframe, you can plant 6 to 8 inches below the surrounding soil level—so the soil will insulate your plants. But digging a pit requires moving a lot of soil, and it makes your coldframe more permanent than you may want.
If you do dig, be aware that rain can run off the frozen ground and into the unfrozen frame bed, causing flooding problems. To prevent this from happening, put a layer of gravel at the bottom of the pit, beneath the layer of soil.
There are other ways to insulate your plants that don’t require a pit. You can pile soil, leaves, or wood chips around the outside of an aboveground frame to hold heat. Or consider adding heat more directly. “Many Alaskans warm their coldframes by putting fresh manure or jugs of water inside,” says Lowenfels. If you go the manure route (creating a hotbed), don’t plant directly in the manure—your plants will burn. Instead, cover the manure with 6 to 8 inches of soil before planting.